Chapter 5: Strongly Enhanced Visible Light Photoelectrocatalytic Hydrogen Evolution
6.3. Results and Discussion
6.3.1. FESEM and EDS Analyses
The morphology and microstructural properties of the as-synthesized F-TiO2 NFs were first studied by FESEM. Fig. 6.1 depicts the FESEM images of the systematic growth of TiO2 NCs on the TiO2 NF surface with the variation of HF concentration from 20 mM to 80 mM, keeping the reaction duration fixed at 8 h.
Fig. 6.1. FESEM images of F-TiO2 NFs: (a) T20, (b) T40; the inset in each case shows the enlarged view of the exposed facets of F-TiO2 NF decorated with self-grown F-TiO2 NCs. (c, d) FESEM images of T20VR and T40VR, respectively. The inset in each case shows the magnified image revealing the mesoporous nature of the TiO2 NF. (e, f) FESEM images of F-TiO2 NFs in T60 and T80, respectively. The inset in each case shows the magnified view of the mesoporous F-TiO2 NFs decorated with self-grown F-TiO2 NCs.
It is clear from Fig. 6.1(a) that for the sample T20, the growth of symmetric TiO2 NFs is complete and the surface of each NF petal/component (truncated tetragonal pyramidal TiO2
nanocrystal1) is covered with uniform self-grown TiO2 NCs due to the irregular surface etching by HF. Here due to selective etching, small sized NCs separated by nanosized pores are created on each surface. When the HF concentration increases from 20 mM to 40 mM, the exposed highly reactive {001} surface gets etched selectively creating cavities on each {001} surface (see Fig. 6.1(b)), while the {101} surface remains relatively smooth covered with TiO2 NCs due to the less reactivity. After the post-growth heat treatment (RTA or furnace annealing) under vacuum, a major change in the surface feature of the TiO2 NFs has been observed from FESEM.
After vacuum RTA, the surface of T20 is transformed into a mesoporous structure with very tiny NCs in between the pores, as shown in Fig. 6.1(c). The FESEM image of T40VR shown in Fig.
6.1(d) depicts the cavity formation in {001} facet and overall mesoporous surface. When the HF concentration is further increased (> 40 mM), the cavity on the {001} facet becomes smaller, and all the surfaces of the TiO2 NFs get etched nearly uniformly irrespective of the nature of the facets, and larger NCs are formed. Theoretical and experimental studies on anatase TiO2 have shown that surface energy (g) of {001} facet is greater than the {101} facet (g{001} (0.90 J m-2) >
g{101} (0.44 J m-2)). Thus, {001} facet is more chemically active than {101}, and it reacts with the HF faster, which creates the cavities on the {001} faceted TiO2 surface. This results in the formation of TiO2 NCs of arbitrary shape and size. It is observed that the size of the TiO2 NCs becomes larger with increasing the HF molar concentration (from 60 mM to 80 mM) (see Fig.
6.1(e, f)).
We have also monitored the growth of TiO2 NFs with the variation of reaction duration from 4 h to 16 h using 20 mM HF solution as solvent (see Fig. 6.2). An incomplete growth of TiO2 NFs is observed for 4 h reaction (see Fig. 6.2(a)), where the NF like structures start evolving. After 6 h of reaction, it is observed that the NFs are developed with truncated tetragonal pyramidal TiO2 NCs, and the NF size becomes larger, as shown in Fig. 6.2(b). After 8 h of reaction, the growth of NFs is completed as shown in Fig. 6.1(a), where a uniform growth of the NF components in all directions with almost identical size is observed. Further increase in reaction duration leads to the over-growth of the NFs. For 12 h of reaction, the number of truncated tetragonal pyramidal TiO2 NFs growth increases and it starts fusing with each other (see Fig. 6.2(c)).
Fig. 6.2. FESEM images of F-TiO2 NFs: (a) T20-4h, (b) T20-6h, (c) T20-12h and (d) T20-16h. The inset in each case shows the enlarged view of the F-TiO2 NF.
After 16 h of reaction, excessive growth is observed, and thus the TiO2 NFs are fused with each other compactly giving it a nearly spherical shape, see Fig. 6.2(d). Similar to T20VR and T40VR, T60VR and T80VR also exhibit a similar type of porous surface feature, as shown in Fig. 6.3(a, b), respectively. However, the sample T20VR shows the optimal mesoporous nature with very tiny NCs uniformly distributed over the surface.
Fig. 6.3. FESEM images of differently annealed F-TiO2 NFs prepared with various HF concentrations: (a) T60VR, (b) T80VR, (c) T20AR and (d) T80AR. The inset in each case shows the enlarged view of the exposed facets of F- TiO2 NF.
After air RTA, T20 and T80 exhibit comparatively less mesoporous nature, which may be due to the lower density of OV defects (See Fig. 6.3(c, d)). During vacuum RTA, oxygen atoms may leave the bulk of the crystals, and as a result, Ov defects are created in high density, which may eventually form Ov clusters/voids inside the TiO2 crystal.8 These voids may be responsible for the evolution of the mesoporous structure in TiO2.9 Particularly, in case of vacuum RTA, the rapid change in thermal environment promotes coalescence of Ov defects and creation of voids as compared to the case of conventional annealing with slow heating/cooling rate. In the case of air RTA, due to the high oxygen partial pressure, the creation of void is not favorable and hence it gives rise to a less mesoporous structure.
To confirm the elemental composition and the doping of TiO2, EDX spectra of the as- grown, as well as the differently treated samples, were recorded and the corresponding atomic percentage of the constituent elements are tabulated in Table 6.2. The EDX spectra of various as-grown TiO2 NFs confirm the presence of Ti, O and F in the nanostructures. In case of T10, TiO2 is formed with a low density of oxygen vacancy (OV) and with 3.5 at% of F. When the HF concentration is increased to 20 mM, the concentration of F reaches to ~7.5 at% with a higher density of OV. Further increase in HF concentration and reaction duration leads to the increase in OV concentration, though the concentration of F in the samples remains almost unaltered (see Table 6.2), which implies that the doping and surface adsorption of F ions reaches to its optimum level beyond the 20 mM HF concentration and 8 h of reaction duration.
Table 6.2. Details of the atomic percentage (at%) of constituent elements in the various samples analyzed from their respective EDX spectra.
Sample code O (at%) Ti (at%) F (at%)
T10 62.3 34.2 3.5
T20 59.2 33.3 7.5
T20-12h 57.2 34.9 7.9
T40 57.0 35.1 7.9
T20VR 53.8 45.3 0.9
T20AR 58.5 39.3 2.2
T20VA 57.9 40.6 1.5
T20AA 60.5 36.7 2.8
It is observed that after the vacuum annealing (RTA and furnace), the OV density increases further, but the density of F ions decreases dramatically (see Table 6.2). The heat treatment at high temperature (600 °C) removes the F adsorbed on the surface of the TiO2 NFs. In the case of
sample T20, after vacuum RTA the OV density increases dramatically and reaches the highest value among all the samples. This effect is found to be less in the sample T20AR (air RTA, oxygen-rich environment). In the case of furnace annealing, the OV density increases from that of the untreated sample, but not as high as in RTA treated sample. Thus, it is clear that the RTA treatment is superior over the normal furnace annealing to create controlled defects in the TiO2
system.